1. Field of the Invention
The present invention relates to castings made from aluminium and its alloys, whereof at least one face has at least one region of wear-resistant zones.
2. Discussion of Background
It is known that castings made from aluminium and its alloys, which are subject to friction, must have mechanical characteristics such as wear-resistance and siezure resistance for at least the "active" face thereof, i.e. the face in contact with an opposing part. Usually these characteristics are only required in regions comprising only a portion of the face and in clearly defined zones of said regions. These zones generally receive surface hardening treatments.
Thus, for example, French patent No. 2,371,520 describes a process for treating a given exposed region of an article made from aluminium or its alloys in the presence or absence of an alloying agent by using a laser beam or an electron beam of high energy (at least 10,000 watts/cm.sup.2) and a diameter between 250 and 1250 .mu.m, so as to be able to heat the article in a first zone and melt it in a second zone within said first zone. Thus, a fine grain structure is obtained, at least in said second zone, with the aim, inter alia, of improving the wear-resistance while eliminating or reducing the deformations resulting from the treatment. Claim 24 of said patent states that "said beam is displaced along several separate, spaced paths, so as to affect the zones forming spaced regions on the exposed surface of said article."
In the same way, French patent No. 2,367,117 claims "a process for treating a metal bearing surface of a component, characterized in that it consists solely of thermally treating bearing zones in order to modify their metallurigical properties and improve their wear-resistance." According to a special example of this process, the treatment is applied to a pig iron cylinder liner or jacket hardened by cementation and consists of directing onto the latter a laser emitting a beam such that it forms a point with a diameter of 3000 .mu.m.
Japanese patent application No. 59,212,572 teaches a treatment process for hardening the inner surface of an internal combustion engine cylinder by means of a laser in which the surface ratio of the untreated zones to the treated zones is preferably between 10 and 70% and the size of the treated zones is several mm.
Thus, these documents teach that zones suitable for friction bearing surfaces can be obtained on castings or members by thermally treating the surface thereof, so as to locally obtain zones with a width exceeding 250 .mu.m.
Applicants have found that it is possible to obtain localized hardening in the treatment zones, but that said hardening in the case of local melting was on the one hand accompanied by a deformation of the casting, so that remachining was required prior to use and in certain cases said hardening was accompanied by cracking and incipient fatigue cracks due to surface stresses resulting from local overheating caused by the treatment.